The present invention relates to electrical stress control and more particularly to a method and an arrangement to control electrical stress in a region of high electric field strength associated with high voltage electrical equipment.
It is well known to employ stress control means to control electrical stress in a region of high electrical field strength due to a shield discontinuity in high voltage cable or electrical equipment, for example, electrical bushings, and joints or terminations of high voltage cables. Such stress control means typically comprise stress cones and tapes or tubular articles of semi-conductive stress control material. This invention is directed to stress control means comprising high permittivity stress control material and provides improved corona discharge extinction, power frequency voltage withstand and impulse voltage withstand performance over prior art arrangements using such stress control means. For purposes of illustration, this invention is described primarily as it applies to a termination of a high voltage cable. The invention can be applied, however, to other electrical cable or equipment where stress control is desired.
A typical high voltage cable includes an inner conductor surrounded by a conductor shield which is, in turn, surrounded by an insulating material that is surrounded by an outer electrically semi-conductive shield, in some instances, and metal shield. In terminating such a cable, it is customary to remove or cut back each successive layer of the cable to expose the layer below. Cutting back the electrically semi-conductive shield causes a discontinuity in the electric field resulting in high electric stress at the end of the shield. The high electrical stress can cause electrical discharges to occur, which in turn tend to cause breakdown of the insulation of the cable. The high electrical stress can be controlled by electrical stress control means.
High-voltage alternating current cable terminations are generally tested in the U.S. under the IEEE standard test procedure Std. 48-1990. This procedure sets forth, inter alia, design tests to be performed by the manufacturer to obtain information on the performance of a high voltage cable termination.
The design tests of the IEEE procedure that are particularly useful in determining the effectiveness of a termination which includes a stress control arrangement include the xe2x80x9cPartial Discharge (Corona) Extinction Voltage Test,xe2x80x9d the xe2x80x9cPower Frequency Voltage Withstand Testxe2x80x9d and the xe2x80x9cLightning Impulse Voltage Withstand Test.xe2x80x9d In the discharge extinction voltage test, electrical discharge in the termination is measured at specific applied voltages and has to be below specific values. Also the voltage at which the discharge extinguishes is measured and has to be above specific values. In the power frequency voltage withstand tests the specified voltage is applied to the cable and should be withstood without flashover or other dielectric breakdown. In the impulse voltage withstand test, impulses of specific value and waveform are applied to the cable and should be withstood without flashover or other dielectric breakdown. The voltage at which flashover occurs should be above specific values. The discharge, power frequency voltage and impulse voltage performance of the termination should meet the requirements set forth in the IEEE Standard Test procedures STD 48-1990.
The use of stress control material in high voltage cable terminations does not always produce terminations that meet the impulse performance requirements of the IEEE test procedures. In order to meet this requirement the stress control arrangement may be augmented by the use of rain sheds. While sheds are typically employed with outdoor terminations for other purposes, they are not generally employed when the cable termination is installed indoors. Since the use of sheds adds to the cost of the termination and requires additional space around the cable, it is desirable to be able to dispense with the use of the sheds yet still meet the desired impulse performance.
The present invention, provides a novel arrangement that significantly improves the termination""s discharge, power frequency voltage and impulse voltage performance with or without the use of sheds. While the present invention is primarily described in connection with a termination of a cable, it is suitable for employment with high voltage cable joints and other high voltage equipment including electrical bushings and feedthroughs.
The present invention includes an elastically recoverable elastomeric insulating sleeve which is provided with an inner support or xe2x80x9ccorexe2x80x9d which holds the sleeve in a stretched condition. The sleeve is placed over the power cable and the core is unwound and removed, allowing the sleeve to contract into contact with the cable. Between the sleeve and the core is disposed a two-part stress control system consisting of a non-tacky, void-filling conformable stress control material surrounded by an elastomeric stress control tube. Both the conformable stress control material and the stress control tube have high permittivity (greater than 10). xe2x80x9cPermittivityxe2x80x9d is synonymous with dielectric constant and is the ratio of electric flux generated by an electrical field in a medium to that generated by the field in a vacuum.
The present invention defines relationships among the permittivities of the conformable stress control material and the stress control tube, the thicknesses of both members, and the length the conformable stress control material extends from the edge of the semi-conductive shield layer of the cable.
In a first embodiment of the invention, conformable stress control material is disposed in contact with the cut end of the cable shield and extends along the cable insulation. In a second embodiment of the invention, a conformable stress control material is also in contact with the cut end of the cable insulation and lug.